Optical microcavities are used for controlling, confining or otherwise passing light for a variety of arrangements and applications such as lasers, optical memory devices, emitters and resonators. Generally, optical microcavities have a relatively thin layer in which light is passed, sometimes referred to as a spacer layer, with reflective material on faces of the thin layer to confine the light. These microcavity layers have dimensions on the scale of several hundred micrometers or less (thus the term “micro” in “microcavities”). In some applications, the microcavity layers are less than about one micrometer in thickness, and as such are sometimes referred to as nanocavities.
One type of microcavity used in many applications is the photonic crystal cavity. Photonic crystal cavities are particularly suited for controlling and confining light in scales on the order of a cubic wavelength of light. Photonic crystal cavities employ a crystal membrane material having a periodic arrangement of holes and exhibiting a photonic band gap, often referred to as a forbidden zone in which light of a particular wavelength range is blocked. Light entering the photonic crystal cavity refracts through and partially reflects off of interfaces between the crystal structure and air at each of the holes. Light is selectively passed through the crystal structure in accordance with the size and arrangement of holes (or lack of holes in certain locations), the light's wavelength, the light's direction of travel, and the refractive index of the crystal structure.
In this regard, by controlling the type of crystal structure and holes therein, photonic crystals can be used to affect the motion of light in a manner similar to the way in which semiconductors affect the motion of electrons. When used with light emitters, photonic crystals facilitate desirable control over the radiative properties of the emitters. High-Q cavities defined in photonic crystals confine photons to a small volume, facilitating large light-matter interaction, which can be useful in applications involving light sources, photodetectors, bio-chemical sensors, and quantum information processors. In these applications, the photonic crystal cavity enhances the generation rate for photons and also facilitates high collection efficiency.
For many crystal emitter applications, room-temperature operation is desirable for a variety of purposes. Certain photonic crystal arrangements such as photonic crystal cavity lasers and LEDs have had some success in achieving room-temperature operation. In addition, emitters such as nitrogen vacancy centers, CdSe quantum dot emitters, and single molecules can be used to generate single photons at room temperature for applications such as fluorescent labeling in biological imaging. However, such emitters are limited in application; as such, many photonic crystal cavity arrangements (e.g., single photon sources for quantum information processing) have relied upon other types of emitters that decay (e.g., cannot operate) at room temperature, often requiring implementation at cryogenic temperatures. Such temperature dependency limits the usefulness of these photonic crystal cavity arrangements and tends to increase the cost of their use.
The usefulness of room-temperature emitters has also been limited due to difficulties in coupling the emitters to acceptable photonic crystal cavities. For instance, many such emitters are in nanocrystalline form and cannot be directly incorporated in a semiconductor or dielectric material, such as that employed by a photonic crystal or other cavity. In addition, emitters operating at visible or near-visible wavelengths have been difficult to combine with cavities employing high index semiconductor material.
In addition to the above, photonic crystal-based lasers (which operate at room temperature) have suffered from inefficiencies due to non-radiative surface recombination losses at exposed semiconductor surfaces. This problem is particularly salient in photonic crystal structures in which quantum wells expose a relatively high surface area. Furthermore, photonic crystal lasers have also been susceptible to heating problems similar to those that have presented challenges to many semiconductor laser applications.
These and other characteristics have presented challenges to the implementation of microcavity arrangements such as photonic crystal cavity arrangements and others.